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Optical Spectroscopy and Photometry of Main-Belt Asteroids with a High

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Optical Spectroscopy and Photometry of Main-Belt Asteroids with a High Research in Astron. Astrophys. 2015 Vol. X No. XX, 000–000 Research in http://www.raa-journal.org http://www.iop.org/journals/raa Astronomy and Astrophysics Optical Spectroscopy and Photometry of Main-Belt Asteroids with a High Orbital Inclination Aya Iwai1, Yoichi Itoh2, Tsuyoshi Terai3, Ranjan Gupta4, Asoke Sen5, Jun Takahashi2 1 National Institute of Advanced Industrial Science and Technology, 1-1-1, Umezono, Tsukuba, Ibaraki 305-8563, Japan 2 Nishi-Harima Astronomical Observatory, Center for Astronomy, University of Hyogo, 407-2, Nishigaichi, Sayo, Hyogo 679-5313, Japan; [email protected] 3 National Astronomical Observatory of Japan, 650 North Aohoku Place, Hilo Hawaii 96720, USA 4 Inter-University Centre for Astronomy and Astrophysics, Ganeshkhind, Pune 411 007, India 5 Department of Physics, Assam University, Silchar 788 001, India Received 2015 xxxx xx; accepted 2015 xxxx xx Abstract We carried out low-resolution optical spectroscopy of 51 main-belt asteroids, most of which have a highly-inclined orbit. They are selected from the D-type candidates in the SDSS-MOC 4 catalog. Using the University of Hawaii 2.2 m telescope and the Inter-University Centre for Astronomy and Astrophysics 2 m telescope in India, we de- termined the spectral types of 38 asteroids. Among them, eight asteroids were classified as D-type asteroids. Fractions of D-type asteroids are 3.0 ± 1.1 % for low orbital inclination main-belt asteroids and 7.3 ± 2.0 % for high orbital inclination main-belt asteroids. We consider that a part of D-type asteroids were formed within the ecliptic region between the main belt and Jupiter, and were then perturbated by Jupiter. Key words: minor planets, asteroids: general — techniques: spectroscopic 1 INTRODUCTION Asteroids with a high orbital inclination are outlier. Terai & Itoh (2011) surveyed small asteroids at high ecliptic latitudes. By using wide and deep optical images taken with Suprime-Cam mounted on the Subaru Telescope, they detected 656 asteroids with a high orbital inclination. The orbital semi-major axes of most of the asteroids were derived to be between 2.0 astronomical units (AU) and 3.3 AU. arXiv:2006.01123v1 [astro-ph.EP] 30 May 2020 Terai, Takahashi & Itoh (2013) found that the cumulative size distribution of the high orbital inclination main-belt asteroids (hereafter MBAs) was shallower than that of the low orbital inclination MBAs. It is considered that MBAs with a high orbital inclination have high collisional velocities. The shallow size distribution for high orbital inclination MBAs indicates that a large body has a higher disruptive strength under hyper-velocity impacts. The size distribution of high orbital inclination asteroids provides us with information regarding the process of planetesimal collision under different conditions from those of low orbital inclination planetesimals. However, the origin of high orbital inclination asteroids is still uncertain. Nagasawa, Tanaka & Ida (2000) calculated the orbital evolution of asteroids. They considered the depletion of gas in the Solar Nebula. An inner edge of the gas was moving outward at a velocity of 1 × 10−5 AU yr−1. It was revealed that the eccentricity and inclination of the asteroids increased due to the motion of secular resonances caused by the gas depletion, while semi-major axis of the asteroids did not change. 2 A. Iwai and Y. Itoh Ida & Makino (1993) investigated the orbital evolution of planetesimals near to a proto-planet. Their numerical simulations showed that the planetesimals moved inward or outward with increasing orbital inclinations and/or eccentricities due to the gravitational perturbation of the proto-planet. In this paper, we focus on D-type asteroids. D-type asteroids are abundant in the Trojan and main- belt population with the semi-major axis beyond ≃ 3 AU. Optical reflectance of the D-type asteroids increases significantly with wavelengths. Such optical reflectance and low albedo of the D-type aster- oids are similar to those of some comet nuclei. Many properties of the D-type asteroids, including its spatial distribution and optical reflectance, have been discussed in the context of cometary evolution and asteroid populations. Detail discussion on the D-type asteroids are found, e.g., in Fitzsimmons et al. (1994). We conducted low-resolution optical spectroscopy of main-belt asteroids. Most of these asteroids have a highly-inclined orbit and optical colors consistent with spectral type D. Comparison of D-type ratio in high and low inclination populations will help us to understand the formation and evolution pro- cess of high inclination asteroids. However, we cannot directly apply the D-type ratio from large volume photometric catalogs, e.g., the SDSS-MOC 4 catalog, because the classification using multi-color pho- tometry is not certain (Carvano et al. 2010, DeMeo & Carry 2013, DeMeo & Carry 2014). Therefore we carried out the spectroscopic observations of confirming the D-type candidates and estimating the real D-type ratio. Hereafter we define low orbital inclination MBAs as the asteroids with the semi-major axis between 2.1 AU and 3.3 AU and with the inclination less than 10 ◦, and high orbital inclination MBAs as the asteroids with the same semi-major axis range with the inclination equal to or larger than 10 ◦. 2 OBSERVATIONS We carried out low-resolution optical spectroscopy of MBAs using the Wide Field Grism Spectrograph 2 (WFGS2) mounted on the University of Hawaii (UH) 2.2 m telescope. The data were obtained on October30 and 31, 2008and October19 and 20, 2011with the low-dispersiongrism 1 and the 1′′. 4 width slit. These instrument settings achieved a wavelength coverageof 440 – 830 nm and a spectral resolution of ∼ 410 at 650 nm. Slit orientations have been fixed in the north-south direction. The telescope was operated in the non-sidereal tracking mode. The integration time for each object was between 180 s and 600 s. Facilities of Inter-University Centre for Astronomy and Astrophysics (IUCAA) were also used. We employed the IUCAA Faint Object Spectrograph and Camera (IFOSC) mounted on the 2 m telescope at the IUCAA Girawali Observatory (IGO), India. The data were obtained on December 28 and 29, 2008 with the IFOSC 5 grism and the 1′′. 5 width slit. The wavelength coverage was 520 – 1030 nm with a spectral resolution of ∼ 650 at 650 nm. Slit orientations have been fixed in the north-south direction. The telescope was operated in the non-sidereal tracking mode. The integration time for each object was between 300 s and 600 s. Several criteria were used to select the targets. First, we selected asteroids with an orbital semi- major axis of between 2.1 AU and 3.3 AU, and with an orbital inclination greater than 10 ◦. We selected high orbital inclination asteroids because lack of the high inclination samples in the previous studies. One may consider that definition of the high and low orbital inclination asteroids should be based on the ν6 resonance. Because the orbital inclination of the ν6 resonance varies with the semi-major axis, clas- sification based on the ν6 resonance may complicate the discussion on the selection bias of the targets. Instead, we simply define the high and low orbital inclination asteroids based on their inclinations. Most of the selected asteroids are listed in the SDSS-MOC 4 catalog and their optical magnitudes are given. We constructed an optical color-color diagram of the selected asteroids using the SDSS magnitudes, and classified those objects with g − r ≥ 0.5 mag and r − i ≥ 0.2 mag as candidates for D-type asteroids (Ivezic et al. 2001). Because we selected the targets on the gri color-color diagram, we considered the selection bias also on the gri color-color diagram (section 4). We were not able to completely fill the observing time whilst searching for D-type asteroids. Therefore, when we could not find any suitable candidates, we observed high orbital inclination as- teroids instead, even if their SDSS colors were unknown. As a result, we observed a total of 48 high Highly-Inclined Asteroids 3 orbital inclination asteroids. We also observed three low orbital inclination asteroids for reference. The optical spectra of dwarfs with a spectral type of G were also taken as spectral standards. After the observations of this work, DeMeo & Carry (2013) proposed a new method for the taxon- omy with using the SDSS g-,r-,i-, and z-band filters. They claimed that the asteroid types are identified with the gri slope and i − z color. The D-type asteroids are classified as the group with the reddest gri slope and the reddest i − z color. The targets in this paper were classified into the D-type, L-type, or the S-type with the large i − z magnitudes by the classification of DeMeo & Carry (2013). Efficiency of the discovery of the D-type asteroids would increase, if we used the classification method of DeMeo & Carry (2013). This will be attempted in future. Additional photometric observations were carried out on November 7, 8, and 9, 2012 with the Multiband Imager for Nayuta Telescope (MINT) mounted on the Nayuta Telescope at Nishi-Harima Observatory, Japan. We observed 4 asteroids whose SDSS magnitudes were unknown. We used SDSS g−, r−, and i−band filters. The field of view was 10 ′× 10 ′with a pixel scale of 0′′. 3. The full width at half maximum of the point spread functions was typically 2′′. 0or2′′. 5. The integration time was between 30 s and 600 s for each object. Since an asteroid rotates and its brightness may vary over time, we repeatedly observed an asteroid in the r−band. The objects we investigated are summarized in table 1. We reduced the spectroscopic data with the following steps: overscan subtraction, bias subtraction, flat fielding, removal of scattered light caused by the telescope and/or the instrument, extraction of a spectrum, and wavelength calibration.
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